Semiconductor quantum dots have received significant attention for biological applications such as cellular imaging, but their constituent toxic elements (e.g., CdSe) and their need for UV excitation can limit their use in vitro and compromise in vivo applications. Rare earth ion-doped nanocrystals, such as rare earth ion-doped yttrium oxide (Y2O3), are an interesting alternative to CdSe quantum dots for two significant reasons: they are nontoxic, and they can be prepared as upconversion materials. In the latter context, they absorb multiple infrared (IR) photons and emit in the visible region.
Because IR excitation is less damaging and penetrates further into living tissue than UV, upconverting nanocrystals are promising materials for in vivo imaging. However, untreated nanocrystals are not without problems: particles tend to aggregate, and they lack surface groups that can be used to attach biomolecular probes. Whereas surface coating with silica/siloxane layers is a common method of enabling particle-biomolecule conjugation, this treatment can significantly increase particle size, which affects transport to and into cells, and silica and siloxane coatings can be hydrolytically unstable under physiological conditions.
There exists a need for a nanoparticle surface treatment yielding a robust, covalently bound, hydrolytically stable organic monolayer capable of being functionalized for bonding to organic molecules that only nominally increased the particle size yet allowed the particle to be chemically bound to a biomolecular probe targeting reagent.